Microfabricated filter and shell constructed with a permeable membrane

ABSTRACT

Microfabricated filters constructed with permeable polysilicon membranes and methods for fabricating such filters. The filters include a frame structure having a plurality of openings therethrough and a permeable polysilicon membrane disposed over the openings in the frame structure. The frame structure provides support for the permeable polysilicon membrane. The pores of the filter may be smaller than the resolution limit of photolithography. The width of the pores may be as small as about 0.01 μm, while the length of the pores may be as small as about 0.05 μm. The filters feature a relatively high throughput due to the extremely short pore length. The filters may be fabricated utilizing standard microfabrication processes. Also, microfabricated shells constructed with permeable membranes for encapsulating microfabricated devices such as microelectromechanical structures (MEMS) and methods for fabricating such shells. The shells include a frame structure having a plurality of openings therethrough, a permeable membrane disposed on the openings through the frame structure, an optional sealing structure disposed on the permeable membrane, and a cavity bounded by the frame structure. The frame structure provides support for the permeable membrane. The permeable membrane may be a thin film layer of polysilicon having a thickness of less than about 0.3 μm. The shells and methods for fabricating the shells minimize the damage incurred by the encapsulated microfabricated device during the fabrication of the shell without restricting the width of the shell. The shells may be fabricated utilizing standard microfabrication processes.

BACKGROUND OF THE INVENTION

The present invention relates generally to filtration devices and moreparticularly to microfabricated filters constructed with permeablemembranes. The present invention further relates to microfabricatedshells constructed with such membranes for encapsulating microfabricateddevices such as microelectromechanical structures (MEMS).

Filtration devices are extensively used in industrial applications, suchas within the biomedical industry, for separating particles of specificsizes from a fluid. For these applications, required filtration devicefeatures typically include: relatively uniform pore sizes anddistributions, pore sizes as small as the nanometer (nm) range, highthroughput, and adequate mechanical strength.

Filter pore sizes in the nanometer range would allowbiologically-important molecules to be mechanically separated on thebasis of size. For instance, such pore sizes may be used to achieve theheretofore elusive goal of viral elimination from biological fluids.

Filters constructed with porous materials are known in the art. Forinstance, a porous polycrystalline silicon (polysilicon) plug for use asa filter is described by Anderson in "Formation, Properties, andApplications of Porous Silicon," Ph.D. Thesis, Dept. of ChemicalEngineering, U.C. Berkeley, April 1991, and summarized in "PorousPolycrystalline Silicon: A New Material for MEMS," Journal ofMicroelectromechanical Systems," Vol. 3, No. 1, March 1994, pp. 10-18.The porous polysilicon plug is formed by depositing a layer ofpolysilicon on a substrate using low-pressure chemical vapor deposition(LPCVD) and then etching the polysilicon layer with an electrochemicalanodization process to make it porous. The porous polysilicon providespore features of about 0.3 micrometers (μm) in width. Theelectrochemical etching process, however, requires an anodizationapparatus, which is not typically used in standard microfabricationprocesses. Furthermore, the porous polysilicon plug is permeable only ina planar direction with respect to the substrate.

The permeability of thin layers (less than about 0.3 μm thick) ofpolysilicon to hydrofluoric (HF) acid has been discussed by Judy et al.in "Polysilicon Hollow Beam Lateral Resonators," Proceedings of the IEEEMicro Electromechanical Systems Workshop, Fort Lauderdale, Fla., Feb.1-10, 1993, pp. 265-71; by Monk et al. in "Stress-corrosion Cracking andBlistering of Thin Polysilicon Films in Hydrofluoric Acid," MaterialsResearch Society Symposium Proceedings, Vol. 308, San Francisco, Calif.,May 1993, pp. 641-6; and by Chonko et al. in "The Integrity of Very ThinSilicon Films Deposited on SiO_(s)," The Physics and Chemistry ofSiO_(S) and the Si--SiO_(s) Interface 2, edited by C. R. Helms, PlenumPress, New York, 1993, pp. 357-62. However, these references are notdirected to the use of thin layers of polysilicon as filters.

Microfabricated shells are used to encapsulate microfabricated devicessuch as MEMS. MEMS include devices such as micro-resonators and inertialsensors. The shells provide a hermetic, low-pressure environment that isessential for achieving a high quality (Q) factor and low Brownian noisein the operation of MEMS.

Microfabricated shells may be fabricated by etching a sacrificial layerdisposed beneath a frame layer, thus forming a cavity, as described byLin in "Selective Encapsulation of MEMS: Micro Channels, Needles,Resonators and Electromechanical Filters," Ph.D. Thesis, ME Department,University of California, Berkeley, Berkeley, Calif., December 1993. Inthis technique, etch holes are formed through the frame layer to allowan etchant to pass into the shell and etch the sacrificial layer. Theetch holes are subsequently closed to hermetically seal the shell bydepositing a sealant over the frame layer.

The etch holes are placed around the perimeter of the frame layer tominimize the amount of sealant passing through the etch holes anddepositing on the encapsulated microfabricated device. Deposition ofsealing film on the microfabricated device is undesirable since it mayalter the device's characteristics. However, this placement of the etchholes increases the time required to etch the sacrificial layer due tothe increased distance the etch is required to travel to remove thesacrificial layer. Long etch times are undesirable since long-termexposure to hydrofluoric acid is damaging to polysilicon structureswhich may be present in the microfabricated device. As a result, thewidth of shells must be limited in order to keep the etch timesreasonable.

The use of permeable polysilicon for fabricating microfabricated shellsis mentioned by Judy in "Micromechanisms Using Sidewall Beams," Ph.D.Thesis, EECS Dept., U.C. Berkeley, May 1994 and by Lin in his 1993 Ph.D.Thesis mentioned above. However, neither reference discloses any detailsof a structure or fabrication process for incorporating permeablepolysilicon in such shells.

Accordingly, it is an object of the present invention to provide filtershaving a pore width as small as the nanometer range, yet also having apore length as small as the tenths of a micrometer range to maximizethroughput.

An additional object of the present invention is to provide filters thathave a high mechanical strength.

A further object of the present invention is to provide methods for theconstruction of such filters using standard microfabrication processes.

Another object of the present invention is to provide shells thatminimize the damage incurred by the encapsulated microfabricated deviceduring the fabrication of the shell without restricting the width of theshell.

Yet another object of the present invention is to provide methods forthe construction of such a shell.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theclaims.

SUMMARY OF THE INVENTION

The present invention is directed to microfabricated filters and methodsfor fabricating such filters. The present invention is further directedto microfabricated shells constructed with permeable membranes forencapsulating microfabricated devices such as MEMS and methods forfabricating such shells.

The microfabricated filters include a frame structure having a pluralityof openings therethrough. A permeable polysilicon membrane is disposedover the openings in the frame structure. The frame layer providessupport for the permeable polysilicon membrane, thus improving themechanical strength of the filter. The plurality of openings in theframe structure may be distributed over the surface of the framestructure.

The construction of such microfabricated filters may begin with a bulksubstrate. A sacrificial structure is then formed over the bulksubstrate to define a cavity. Next, a frame structure having a pluralityof openings is formed over at least part of the sacrificial structureand the bulk substrate. A permeable polysilicon structure is then formedover at least part of the frame structure. Finally, the sacrificialstructure is removed with an etchant. The permeable polysiliconstructure allows the etchant to pass through the openings in the framelayer and etch the sacrificial structure, thus forming the cavity.

The pores of the microfabricated filters are defined by the structure ofthe permeable polysilicon membrane. As a result, the width and length ofthe pores may be smaller than the resolution limit of photolithography.The width of the pores may be as small as about 0.01 μm, while thelength of the pores may be as small as about 0.05 μm.

The filters feature a high throughput due to the extremely short porelength. The filters also provide a relatively high mechanical strengthdue to the support of the permeable membrane by the frame structure. Thefilters may be constructed utilizing standard microfabricationprocesses.

The shells of the present invention are comprised of a bulk substrate, aframe structure having a plurality of openings therethrough disposed onthe bulk substrate, a permeable membrane disposed on the openingsthrough the frame structure, a sealing structure disposed on thepermeable membrane, and a cavity bounded by the bulk substrate and theframe structure. Optionally, a microfabricated device may be disposedwithin the cavity. The frame layer provides support for the permeablemembrane, thus improving the mechanical strength of the shell. Theplurality of openings in the frame structure may be distributed over thesurface of the frame structure to maximize the etch rate of asacrificial layer used to define the cavity. The sealing structurehermetically seals the shell and may be omitted if the shell is intendedfor filtration purposes. The permeable membrane may be a thin film layerof polysilicon having a thickness of less than about 0.3 μm.

The construction of such microfabricated shells may begin with a bulksubstrate. A sacrificial structure is then formed over the bulksubstrate to define a cavity. Next, a frame structure having a pluralityof openings is formed over at least part of the sacrificial structureand the bulk substrate. A permeable membrane is then formed over atleast part of the frame structure. Next, the sacrificial structure isremoved by passing an etchant through the permeable membrane, thusforming the cavity. A sealing layer is then formed over the frame layerand openings, thus hermetically sealing the shell. The sealing layer maybe omitted if the shell is intended for filtration purposes.

The shells and methods for fabricating the shells minimize the damageincurred by the encapsulated microfabricated device during thefabrication of the shell without restricting the size of the shell. Thepermeable membrane allows the etchant to enter the shell while blockingpassage of the sealing layer. As a result, the openings in the framestructure may be distributed over the surface of the frame layer tomaximize the etch rate of the sacrificial structure without causing thedeposition of the sealing layer on the microfabricated device. Theshells may be fabricated utilizing standard microfabrication processes.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, schematically illustrate the presentinvention and, together with the general description given above and thedetailed description given below, serve to explain the principles of theinvention.

FIG. 1 is a perspective view of a filter in accordance with the presentinvention.

FIG. 2 is an enlarged perspective view of the circled area of FIG. 1.

FIG. 3 is a cross-sectional view along line 3--3 of FIG. 2.

FIGS. 4-9 are cross-sectional views illustrating the steps in thefabrication of the filter of FIG. 1.

FIG. 10 is a perspective view of an alternative embodiment of a filterin accordance with the present invention.

FIG. 11 is a cross-sectional view along line 11--11 of FIG. 10.

FIGS. 12-17 are cross-sectional views illustrating the steps in thefabrication of the filter of FIG. 10.

FIG. 18 is a perspective view of a shell in accordance with the presentinvention.

FIG. 19 is a cross-sectional view along line 19--19 of FIG. 18.

FIGS. 20-27 are cross-sectional views illustrating the steps in thefabrication of the shell of FIG. 18.

FIG. 28 is a perspective view of an alternative embodiment of a shell inaccordance with the present invention.

FIG. 29 is a cross-sectional view along line 29--29 of FIG. 28.

FIGS. 30-37 are cross-sectional views illustrating the steps in thefabrication of the shell of FIG. 28.

DESCRIPTION OF THE REPRESENTATIVE EMBODIMENTS

The present invention is directed to microfabricated filters and shellsconstructed with permeable membranes and methods for fabricating suchfilters and shells. The present invention will be described in terms ofseveral representative embodiments.

A filter 100 in accordance with the present invention is shown in FIGS.1, 2 and 3. The filter may be used for separating particles of specificsizes from a fluid.

Referring to FIGS. 1, 2 and 3, filter 100 includes a bulk substrate 101,a frame structure 102 having a plurality of openings 103 therethroughdisposed over the bulk substrate, a permeable polysilicon membrane 104disposed over the frame structure, a cavity 105 bounded by the bulksubstrate and the frame structure, a channel 106, and an inlet/outletport 107. The plurality of openings 103 may be distributed over thesurface of frame structure 102. Openings 103 may, for instance, besquare in shape and range from about 0.3 μm to about 600 μm in width (W)and length (L). To achieve the desired permeability characteristics, thethickness of the permeable polysilicon membrane should be less thanabout 0.3 μm and may be as small as about 0.05 μm.

The flow of fluid through the filter is indicated by arrows 108. Thefluid flow may also occur in the direction opposite to that indicated.

Referring to FIG. 4, fabrication of filter 100 may begin with planarbulk substrate 101 such as a single crystalline <100>-silicon wafer. Asacrificial layer 201 is then deposited on the substrate using LPCVD.The sacrificial layer may be, for instance, a 5 μm-thick layer ofphosphosilicate glass (PSG) containing 8 wt % phosphorus. The PSG may bedeposited, for instance, using a temperature of 450° C., a pressure of300 mTorr, a flow rate of 60 sccm of silane gas (SiH₄), 90 sccm ofoxygen gas (O₂), and 10.3 sccm of phosphene gas (PH₃) for 1.5 hours.

Next, sacrificial layer 201 may be densified by placing bulk substrate101 in, for instance, a nitrogen (N₂) environment at 950° C. for 1 hour.

Referring to FIG. 5, sacrificial layer 201 is then photolithographicallypatterned and isotropically etched to form mold 202. The etch may beperformed using, for instance, a 5:1 buffered HF acid solution at 27° C.for about 3 minutes. Mold 202 is used to define the shape of cavity 105,channel 106, and inlet/outlet port 107 of filter 100 that are formed insubsequent steps of the process.

Next, referring to FIG. 6, a frame layer 203 is deposited over mold 202and bulk substrate 101 using LPCVD. The frame layer may be, forinstance, a 1 μm-thick layer of low-stress silicon nitride (SiN). Theprocess parameters for the deposition may be, for instance: 835° C., 140mTorr, 100 sccm dichlorosilane (DCS), and 25 sccm ammonia gas (NH₃), 4hours.

Referring to FIG. 7, a plurality of openings 204 are thenphotolithographically defined and plasma etched through frame layer 203.Openings 204 may, for instance, be square in shape and have a width (W)ranging from about 0.3 μm to about 600 μm. The plasma etch may beperformed, for instance, with a SF₆ plasma at a chamber pressure of 150mTorr, a radio-frequency (RF) power of 200 Watts, and a gas flow rate of80 sccm for 10 minutes. Frame layer 203 with openings 204 form framestructure 102 of filter 100. An additional opening (not shown) may alsobe etched through frame layer 203 to begin formation of inlet/outletport 107 of filter 100.

Next, referring to FIG. 8, a permeable polysilicon layer 205 isdeposited over frame layer 203 and openings 204 using a thin filmdeposition process, such as LPCVD. To achieve the desired permeabilitycharacteristics, the thickness of the permeable polysilicon layer shouldbe less than about 0.3 μm and may be as small as about 0.05 μm. Theprocess parameters for the deposition may be, for instance: 605° C., 555mTorr, 125 sccm SiH₄, and 15 minutes, forming a permeable polysiliconlayer about 0.1 μm thick. Permeable polysilicon layer 205 formspermeable polysilicon membrane 104 of filter 100.

Permeable polysilicon layer 205 may then be annealed by placing bulksubstrate 101 in, for instance, an N₂ environment at 950° C. for 1 hour.

Next, referring to FIG. 9, mold 202 is removed using an etchant. Theetchant passes through permeable polysilicon layer 205 in openings 204to etch mold 202. The etch may, for instance, be performed withconcentrated HF acid at 27° C. for 2 minutes. This etching step formscavity 105, channel 106, and inlet/outlet port 107 of filter 100.

The processed substrate 101 is then rinsed in deionized (DI) water toremove all remaining HF acid. The rinse may be performed, for instance,for two hours.

Finally, the processed substrate 101 is dried using, for example, asuper-critical carbon dioxide (CO₂) process. This process is selected toprevent the permeable polysilicon layer from cracking during drying.

A filter 300, which is an alternative embodiment of the presentinvention, is shown in FIGS. 10 and 11. Filter 300 includes a bulksubstrate 301, a sacrificial structure 302 disposed over the bulksubstrate, a frame structure 303 having a plurality of openings 304therethrough disposed over the sacrificial structure, a permeablepolysilicon membrane 305 disposed over the frame structure, and aninlet/outlet port 306. The plurality of openings 304 may be distributedover the surface of frame structure 303. openings 304 may, for instance,be square in shape and range from about 0.3 μm to about 600 μm in width(W₁) and length (L₁). To achieve the desired permeabilitycharacteristics, the permeable polysilicon membrane should be less thanabout 0.3 μm in thickness and may be as thin as about 0.05 μm.

The flow of fluid through the filter is indicated by arrow 307. Thefluid flow may also occur in the direction opposite to that indicated.

Referring to FIG. 12, fabrication of filter 300 may begin with planarbulk substrate 301 such as a single crystalline <100>-silicon wafer. Asacrificial layer 401 is then deposited on the substrate using LPCVD.The sacrificial layer may be, for instance, a 5 μm-thick layer ofphosphosilicate glass (PSG) containing 8 wt % phosphorus. The PSG may bedeposited, for instance, using the following parameters: 450° C., 300mTorr, 60 sccm SiH₄, 90 sccm O₂, 10.3 sccm PH₃, and 1.5 hours.

Next, sacrificial layer 401 may be densified by placing bulk substrate301 in, for instance, an N₂ environment at 950° C. for 1 hour.

Referring to FIG. 13, a frame layer 402 is then deposited oversacrificial layer 401 using LPCVD. The frame layer may be, for instance,a 1 μm-thick layer of low-stress SiN. The process parameters for thedeposition may be, for instance: 835° C., 140 mTorr, 100 sccm DCS, 25sccm NH₃, and 4 hours.

Next, referring to FIG. 14, a plurality of openings 403 arephotolithographically defined and plasma etched through frame layer 402.Openings 403 may, for instance, be square in shape and have a width (W₁)ranging from about 0.3 μm to about 600 μm. The plasma etch may beperformed, for instance, with a SF₆ plasma at a chamber pressure of 150mTorr, an RF power of 200 Watts, and a gas flow rate of 80 sccm for 10minutes. Frame layer 402 with openings 403 form frame structure 303 offilter 300.

Referring to FIG. 15, a permeable polysilicon layer 404 is thendeposited over frame layer 402 and openings 403 using a thin filmdeposition process, such as LPCVD. To achieve the desired permeabilitycharacteristics, the thickness of the permeable polysilicon layer shouldbe less than about 0.3 μm and may be as small as about 0.05 μm. Theprocess parameters for the deposition may be, for instance: 605° C., 555mTorr, 125 sccm SiH₄, and 15 minutes, forming a permeable polysiliconlayer about 0.1 μm thick. Permeable polysilicon layer 404 formspermeable polysilicon membrane 305 of filter 300.

Next, permeable polysilicon layer 404 may be annealed by placing bulksubstrate 301 in, for instance, an N₂ environment at 950° C. for 1 hour.

Referring to FIG. 16, inlet/outlet port 306 is then formed byphotolithographically patterning and anisotropically etching thebackside of substrate 301 through to sacrificial layer 401. The etch maybe performed, for instance, using ethylene diamine pyrocathecol (EDP) at110° C. for 10 hours.

Next, referring to FIG. 17, sacrificial layer 401 is then partiallyremoved using an etchant. The etchant passes through permeablepolysilicon layer 404 in openings 403 and through inlet/outlet port 306to etch sacrificial layer 401. The etch may, for instance, be performedwith concentrated HF acid at 27° C. for 2 minutes. This etching stepexposes permeable polysilicon layer 404 to inlet/outlet port 306, thusenabling fluid to flow through filter 300.

The processed substrate 301 is then rinsed in DI water to remove allremaining HF acid. The rinse may be performed, for instance, for twohours.

Finally, the processed substrate 301 is dried using, for example, asuper-critical CO₂ process. This process is selected to prevent thepermeable polysilicon layer from cracking during drying.

Combinations of materials different from those described above may beused to fabricate filters 100 and 200. For instance, sacrificial layers201 and 401 may be composed of low-temperature oxide (LTO), frame layers203 and 402 may be composed of undoped polysilicon, and permeablepolysilicon layers 205 and 404 may be composed of in-situ dopedpolysilicon.

A shell 500 in accordance with the present invention is shown in FIGS.18 and 19. The shell may be used to encapsulate a microfabricated devicesuch as a MEMS. MEMS include devices such as micromachined resonators(microresonators) and inertial sensors. The shell may be used to providea hermetic seal or alternatively, as a filter which selectively allowsthe passage of particles into the shell based on their size.

Referring to FIGS. 18 and 19, shell 500 includes a bulk substrate 501, aframe structure 502 having a plurality of openings 503 therethroughdisposed over the bulk substrate, a permeable membrane 504 disposed overthe frame structure, a sealing structure 505 disposed over the permeablemembrane, a cavity 506 bounded by the bulk substrate and framestructure, and an optional nicrofabricated device 507 disposed withinthe cavity. The microfabricated device may be, for instance, a MEMS. Theplurality of openings 503 may be distributed over the surface of framestructure 502. Openings 503 may, for instance, be square in shape andrange from about 0.3 μm to about 600 μm in width (W₂) and length (L₂).Permeable membrane 504 may be a thin film composed of polysilicon. Toachieve the desired permeability characteristics, the thickness of thepermeable polysilicon membrane should be less than about 0.3 μm and maybe as small as about 0.05 μm. Sealing structure 505 hermetically sealsthe shell and may be omitted if the shell is intended for filtrationpurposes.

Referring to FIG. 20, fabrication of shell 500 may begin with planarbulk substrate 501 such as a single crystalline <100>-silicon wafer.Next, microfabricated device 507, such as a microresonator, mayoptionally be formed on substrate 501 by processes commonly known in theart.

Referring to FIG. 21, a sacrificial layer 602 is then deposited over thesubstrate and the microfabricated device using LPCVD. The sacrificiallayer may be, for instance, a 5 μm-thick layer of phosphosilicate glass(PSG) containing 8 wt % phosphorus. The PSG may be deposited, forinstance, using the following parameters: 450° C., 300 mTorr, 60 sccmSiH₄, 90 sccm O₂, 10.3 sccm PH₃, and 1.5 hours.

Next, sacrificial layer 602 may be densified by placing bulk substrate501 in, for instance, an N₂ environment at 950° C. for 1 hour.

Referring to FIG. 22, sacrificial layer 602 is thenphotolithographically patterned and isotropically etched to form mold603. The etch may be performed using, for instance, a 5:1 buffered HFacid solution at 27° C. for 3 minutes. Mold 603 is used to define theshape of cavity 506 of filter 500 that is formed in subsequent steps ofthe process.

Next, referring to FIG. 23, a frame layer 604 is deposited over mold 603and bulk substrate 501 using LPCVD. The frame layer may be, forinstance, a 1 μm-thick layer of low-stress SiN. The process parametersfor the deposition may be, for instance: 835° C., 140 mTorr, 100 sccmDCS, 25 sccm NH₃, and 4 hours.

Referring to FIG. 24, a plurality of openings 605 are thenphotolithographically defined and plasma etched through frame layer 604.Openings 605 may, for instance, be square in shape and have a width (W₂)ranging from about 0.3 μm to about 600 μm. The plasma etch may beperformed, for instance, with a SF₆ plasma at a chamber pressure of 150mTorr, an RF power of 200 Watts, and a gas flow rate of 80 sccm for 10minutes. Frame layer 604 with openings 605 form frame structure 502 ofshell 500.

Next, referring to FIG. 25, a permeable layer 606 is deposited overframe layer 604 and openings 605 using LPCVD. The permeable layer may,for instance, be a thin film of polysilicon. To achieve the desiredpermeability characteristics, the thickness of the permeable polysiliconlayer should be less than about 0.3 μm and may be as small as about 0.05μm. The process parameters for the deposition may be, for instance: 605°C., 555 mTorr, 125 sccm SiH₄, and 15 minutes, forming a permeablepolysilicon layer about 0.1 μm thick. Permeable layer 606 formspermeable membrane 504 of shell 500.

Permeable layer 606 may then be annealed by placing bulk substrate 501in, for instance, an N₂ environment at 950° C. for 1 hour.

Next, referring to FIG. 26, mold 603 is then removed using an etchant.The etchant passes through permeable layer 606 in openings 605 to etchmold 603. The etch may, for instance, be performed with concentrated HPacid at 27° C. for 2 minutes. This etching step forms cavity 506 ofshell 500.

The processed substrate 501 is then rinsed in DI water to remove allremaining HF acid. The rinse may be performed, for instance, for twohours.

Next, the processed substrate 501 is dried using, for example, asuper-critical CO₂ process. This process is selected to prevent thepermeable layer from cracking during drying.

Finally, referring to FIG. 27, a sealing layer 607 is deposited overpermeable layer 606 using LPCVD. The sealing layer may be, for instance,a 0.8 μm-thick layer of low-stress SiN. The process parameters for thedeposition may be, for instance, 835° C., 140 mTorr, 100 sccm DCS, and25 sccm NH₃. This step forms sealing structure 505 of shell 500 and maybe omitted if the shell is intended for use as a filter rather than as ahermetic seal.

A shell 700, which is an alternative embodiment of the presentinvention, is shown in FIGS. 28 and 29. Shell 700 includes a bulksubstrate 701; a sacrificial structure 702 disposed over the bulksubstrate; a frame structure 703 having a plurality of openings 704therethrough disposed over the sacrificial structure; a permeablemembrane 705 disposed over the frame structure; a sealing structure 706disposed over the permeable polysilicon membrane; a plurality ofopenings 708 disposed through the frame structure, permeable polysiliconmembrane, and sealing layer; and a cavity 709 bounded by the bulksubstrate and the frame structure. Optionally, shell 700 may include ametallization layer 707 disposed over the sealing structure and amicrofabricated device 710 disposed within the cavity. Themicrofabricated device may be, for instance, a microresonator. Openings704 may be distributed over the surface of frame structure 703. Openings704 may be square in shape and range from about 0.3 μm to about 600 μmin width (W₃) and length (L₃). Permeable membrane 705 may be a thin filmcomposed of polysilicon. To achieve the desired permeabilitycharacteristics, the thickness of the permeable polysilicon membraneshould be less than about 0.3 μm and may be as small as about 0.05 μm.Sealing structure 706 hermetically seals the shell and may be omitted ifthe shell is intended for filtration purposes. Metallization layer 707may be used to form external electrical connections to microfabricateddevice 710 through openings 708.

Referring to FIG. 30, fabrication of shell 700 may begin with planarbulk substrate 701 such as a single crystalline <100>-silicon wafer.Next, microfabricated device 710 may be formed on substrate 701 byprocesses commonly known in the art.

Referring to FIG. 31, a sacrificial layer 802 is then deposited over thesubstrate and the microfabricated device using LPCVD. The sacrificiallayer may be, for instance, a 5 μm-thick layer of phosphosilicate glass(PSG) containing 8 wt % phosphorus. The PSG may be deposited, forinstance, using the following parameters: 450° C., 300 mTorr, 60 sccmSiH₄, 90 sccm O₂, 10.3 sccm PH₃, and 1.5 hours.

Next, sacrificial layer 802 may be densified by placing bulk substrate701 in, for instance, an N₂ environment at 950° C. for 1 hour.

Referring to FIG. 32, a frame layer 803 is then deposited oversacrificial layer 802 using LPCVD. The frame layer may be, for instance,a 1 μm-thick layer of low-stress SiN. The process parameters for thedeposition may be, for instance: 835° C., 140 mTorr, 100 sccm DCS, 25sccm NH₃, and 4 hours.

Referring to FIG. 33, a plurality of openings 804 are thenphotolithographically defined and plasma etched through frame layer 803.Openings 804 may, for instance, be square in shape and have a width (W₃)ranging from about 0.3 μm to about 600 μm. The plasma etch may beperformed, for instance, with a SF₆ plasma at a chamber pressure of 150mTorr, an RF power of 200 Watts, and a gas flow rate of 80 sccm for 10minutes. Frame layer 803 with openings 804 form frame structure 703 ofshell 700.

Next, referring to FIG. 34, a permeable layer 805 is deposited overframe layer 803 and openings 804 using LPCVD. The permeable layer may,for instance, be a thin film of polysilicon. To achieve the desiredpermeability characteristics, the thickness of the permeable polysiliconlayer should be less than about 0.3 μm and may be as small as about 0.05μm. The process parameters for the deposition may be, for instance: 605°C., 555 mTorr, 125 sccm SiH₄, and 15 minutes, forming a permeablepolysilicon layer about 0.1 μm thick. Permeable polysilicon layer 805forms permeable polysilicon membrane 705 of shell 700.

Permeable layer 805 is then annealed by placing bulk substrate 701 in,for instance, an N₂ environment at 950° C. for 1 hour.

Next, referring to FIG. 35, sacrificial layer 802 is then partiallyremoved using an etchant. The etchant passes through permeablepolysilicon layer 803 in openings 804 to etch regions of sacrificiallayer 802 underneath openings 804. The etch may, for instance, beperformed with concentrated HF acid at 27° C. for 3 minutes. Thisetching step forms cavity 709 of shell 700.

The processed substrate 701 is then rinsed in DI water to remove allremaining HF acid. The rinse may be performed, for instance, for twohours.

Next, the processed substrate 701 is dried using, for example, asuper-critical CO₂ process. This process is selected to prevent thepermeable layer from cracking during drying.

Referring to FIG. 36, a sealing layer 806 is deposited over permeablelayer 805 using LPCVD. The sealing layer may be, for instance, a 0.8μm-thick layer of low-stress SiN. The process parameters for thedeposition may be, for instance: 835° C., 140 mTorr, 100 sccm DCS, and25 sccm NH₃. This step forms sealing structure 706 of shell 700 and maybe omitted if the shell is intended for filtration purposes rather forforming a hermetic seal.

Next, referring to FIG. 37, openings 708 are then photolithographicallydefined and plasma etched through sealing layer 806, permeable layer805, frame layer 803, and sacrificial layer 802. The plasma etch may beperformed, for instance, with a SF₆ plasma at a chamber pressure of 150mTorr, an RF power of 200 Watts, and a gas flow rate of 80 sccm for 10minutes.

Finally, also referring to FIG. 37, a metallization layer 808 mayoptionally be deposited and defined over sealing layer 806 by processescommonly known in the art. This step forms metallization structure 707of shell 700.

Combinations of materials different from those described above may beused to fabricate shells 500 and 700. For instance, sacrificial layers602 and 802 may be composed of LTO, frame layers 604 and 803 may becomposed of undoped polysilicon, and permeable layers 606 and 805 may becomposed of in-situ doped polysilicon.

The present invention has been described in terms of representativeembodiments. The invention, however, is not limited to the embodimentsdepicted and described. Rather, the scope of the invention is defined bythe appended claims.

What is claimed is:
 1. A microfabricated filter, comprising:a framestructure having a plurality of openings therethrough; and a permeablepolycrystalline silicon membrane disposed over said plurality ofopenings in said frame structure.
 2. The microfabricated filter of claim1, wherein said permeable polycrystalline silicon membrane is formedusing thin film deposition processes exclusively.
 3. The microfabricatedfilter of claim 1, wherein said permeable polycrystalline siliconmembrane has a thickness of between about 0.05 micrometers and about 0.3micrometers.
 4. A microfabricated filter, comprising:a permeablepolycrystalline silicon thin film membrane having a surface, whereinsaid permeable polycrystalline silicon thin film membrane is permeablealong a direction perpendicular to said surface; and a frame adapted toreceive the permeable polycrystalline silicon thin membrame.
 5. Themicrofabricated filter of claim 4, wherein said permeablepolycrystalline silicon thin film membrane has a thickness of betweenabout 0.05 micrometers and about 0.3 micrometers.
 6. A microfabricatedfilter, comprising:a bulk substrate; a frame structure having aplurality of openings therethrough disposed over said bulk substrate; apermeable polycrystalline silicon membrane disposed over said framestructure; a cavity bounded by said bulk substrate and said framestructure; and a port coupled to said cavity.
 7. The microfabricatedfilter of claim 6, wherein said permeable polycrystalline siliconmembrane has a thickness of between about 0.05 micrometers and about 0.3micrometers.
 8. A microfabricated shell, comprising:a frame structurehaving a plurality of openings therethrough; a permeable membranedisposed on said openings through said frame structure; and a cavitybounded by said frame structure.
 9. The microfabricated shell of claim8, wherein said permeable membrane is composed of permeablepolycrystalline silicon.
 10. The microfabricated shell of claim 9,wherein said permeable membrane has a thickness of between about 0.05micrometers and about 0.3 micrometers.
 11. The microfabricated shell ofclaim 8, wherein a microfabricated device is disposed within saidcavity.
 12. A microfabricated shell, comprising:a bulk substrate; aframe structure having a plurality of openings therethrough disposed onsaid bulk substrate; a permeable membrane disposed on said openingsthrough said frame structure; a sealing structure disposed on saidpermeable membrane; and a cavity bounded by said bulk substrate and saidframe structure.
 13. The microfabricated shell of claim 12, wherein saidpermeable membrane is composed of permeable polycrystalline silicon. 14.The microfabricated shell of claim 13, wherein said permeable membranehas a thickness of between about 0.05 micrometers and about 0.3micrometers.
 15. The microfabricated shell of claim 13, wherein amicrofabricated device is disposed within said cavity.